1. Field of the Invention
The present invention relates to a method of light exposure applicable to semiconductor lithography, or a like process, particularly to a method of light exposure utilizing evanescent light.
2. Related Background Art
In recent years, techniques for making a fine semiconductor device are being developed rapidly. Demagnifying light projection methods are employed for transferring a pattern of IC (integrated circuit) or the like by light exposure. In such a method, a mask is prepared for pattern formation; this mask is illuminated by a light projection system; and the light penetrating through the mask is focused by a demagnification lens on a light-receiving base plate to transfer the pattern on the mask onto the light-receiving base plate.
In this method, the minimum dimension of the pattern is limited by occurrence of light diffraction. The limit is about 0.6 .lambda./NA (where .lambda. is light wavelength, and NA is numerical aperture of the lens).
For obtaining a smaller pattern size, a phase shift light projection method is disclosed in Japanese Patent Application Laid-Open No. 62-50811. For lowering the resolution limit, an oblique light projection method is disclosed in Japanese Patent Application Laid-Open No. 4-267515.
Even with the above techniques and a stepper employing an excimer laser light source, the pattern size cannot be made smaller than about 0.15 .mu.m at present.
For lowering the limit caused by light diffraction, a method is disclosed in Japanese Patent Publication Gazette No. 8-179493 (1987), in which so-called evanescent light leaking from a dielectric material is utilized to transfer a pattern of 0.15 .mu.m or finer.
The light exposure employing the evanescent light is explained by reference to FIG. 1. In FIG. 1, laser beam 30 is introduced at such an incident angle that the laser beam is totally reflected as reflected laser beam 31 at the bottom face of prism 32. A mask 34 is provided on glass base plate 33. This mask 34 is made from an electroconductive material such as chromium, and serves as a light-intercepting film. Photosensitive material (photoresist) 36 is applied onto transfer-receiving base plate 37. Glass base plate 33 carrying mask 34 is closely attached to prism 32 with interposition of spacer 38. Thereby, evanescent light 35 leaks through openings of mask 34. When photosensitive material 36 applied on transfer-receiving base plate 37 is brought close thereto to expose it to leaking light 35, a latent image is formed in the photosensitive material by photopolymerization, crosslinking reaction, or a like process. This latent image is developed and fixed to transfer the pattern of the openings of mask 34 to form a fine pattern of not larger than 0.2 .mu.m.
The above prior art techniques, however, utilize a circularly polarized laser beam introduced in one direction, so that they have a disadvantage that the transfer accuracy differs between a vertical pattern and a lateral pattern. More specifically, the mask pattern for an IC or the like is generally constituted of slit-shaped openings arranged perpendicularly to each other, vertically and laterally. Therefore, when light is introduced to a fine slit of not more than the wavelength of the light, the behavior of the light leaking from the slit-shaped openings is different between the linearly polarized light introduced perpendicularly to the length direction of the opening and that introduced parallel thereto. The linearly polarized light introduced perpendicularly to the length direction of the slit leaks in the same shape as the slit, whereas the linear polarized light introduced parallel thereto leaks out along the edge of the opening, not to transfer precisely the pattern of the mask.
In the above prior art techniques, a triangular prism is used. In the triangular prism, the light reflected at the bottom face leaves the prism to the air through the prism face nearly perpendicular to the light path, so that a part of the light to be emitted outside is reflected by the prism face to return to the bottom to cause interference with the original incident light, thereby disturbing the uniform distribution of the evanescent light, disadvantageously.